For each plant species, there exists a wide discrepancy in plant growth due to environmental conditions. Under most conditions, the maximum growth potential of a plant is not realized. Plant breeding has demonstrated that a plant's resources can be redirected to individual organs to enhance growth.
Genetic engineering of plants, which entails the isolation and manipulation of genetic material, e.g., DNA or RNA, and the subsequent introduction of that material into a plant or plant cells, has changed plant breeding and agriculture considerably over recent years. Increased crop food values, higher yields, feed value, reduced production costs, pest resistance, stress tolerance, drought resistance, the production of pharmaceuticals, chemicals and biological molecules as well as other beneficial traits are all potentially achievable through genetic engineering techniques.
The ability to manipulate gene expression provides a means of producing new characteristics in transformed plants. For example, the ability to increase the size of a plants root system would permit increased nutrient assimilation from the soil. Moreover, the ability to increase leaf growth would increase the capacity of a plant to assimilate solar energy. Obviously, the ability to control the growth of an entire plant, or specific target organs thereof would be very desirable.
Receptors located in the plasma membrane play a prominent role in cell signaling. Recently, evidence has suggested that plant cells also carry cell surface receptors that also have protein kinase activity. The plant receptor like protein kinases (RLKs) are structurally related to the polypeptide growth factor receptors of mammals. These proteins have a large extra cytoplasmic domain, a membrane spanning segment and a cytoplasmic domain which has protein kinase activity. Most mammalian growth factor receptor protein kinases are tyrosine kinases, but many plant RLKs are serine/threonine protein kinases. The RLKs have been placed into three categories based on structural similarities: (1) S-domain proteins are related to the self-incompatibility locus glycoproteins of Brassica, (2) leucine-rich repeat proteins contain a tandemly repeated motif that has been found in numerous eukaryotic proteins, and (3) proteins which contain epidermal growth factor-like repeats. The RLKs have been found in monocotyledonous plants such as maize and in dicotyledonous plants such as Brassicaceae (for review see: Walker, J. C., 1994, "Structure and function of the receptor-like protein kinases of higher plants," Plant Mol. Biol. 26:1599-1609). The present invention provides a novel RLK, termed receptor-like protein kinase (RKN).